Currently, because no current flows in the D-type switching amplifier when there is no input signals and also because the amplifier can be rapidly turned on and off when it is switched on so that the amplifier of such type is advantageous in low energy consumption and high efficiency the D-type switching amplifier is appreciated more and more to be a switching device.
The D-type switching amplifier conventionally adopts a structure of combination of N-MOS (N-type-Metal-Oxide-Semiconductor) switch transistor and P-MOS (P-type-Metal-Oxide-Semiconductor) switch transistor. However, since a P-MOS switch tube has a defect of large area, the D-type switching amplifier of CMOS (Complementary-Metal-Oxide-Semiconductor) switch transistor is not adaptive to the minimization for the compact electronic devices. On the other hand, because N-MOS switch transistor can be used as P-MOS switch transistor, and is advantageous in small area and small turn-on resistance per unit area, the D-type switching amplifier adopts an all N-MOS switch transistor structure to replace the switch transistor structure which is based on the combination of P-MOS and N-MOS for reducing the surface area.
However, when such a structure of all N-MOS switch transistor is adopted, a gate driving voltage that is higher than the power supply of the amplifier is required in view of the source terminal of the switch transistor being connected to the power supply of the entire amplifier. For this reason, a bootstrap potential is required to be generated within the chip in order for generating an operation voltage required by the driving circuit.
A schematic diagram of a D-type switching amplifier having a driving circuit in the prior art is shown in FIG. 1. As shown in FIG. 1, Va, Vb represent the input terminals (including logic circuits, or further level conversion circuits), which receive the input pulse-width modulation (PWM) signals having reversed logic, and M1 and M2 are switch transistors or switching circuits. In order to obtain from the output terminal the pulse-width modulation signals corresponding to those at Va, Vb, it is required that the switch transistors M1 and M2 switch between turn-on and turn-off state. Wherein, the bootstrap potential Vh is generated by a bootstrap circuit, which is composed of a circuit for supplementing energy (normally, a diode Dboot), a circuit for storing the energy (capacitor Cboot), and a switch transistor M1 having the function for switching.
The switch transistors M1, M2 of the amplifier are driven by driving circuits I1, I2 respectively. Vd is the operation power supply for driving circuits I1, I2. In order to ensure that switch transistor M1 of the amplifier fully turns on, the output voltage Vout must approximate the power supply voltage Vcc, which in turn makes the driving voltage Vd be larger than the power supply voltage Vcc. This is implemented via the bootstrap potential Vh.
Meanwhile, in order to prevent both the upper and lower switch transistors M1 and M2 from being damaged because of a large current which flows directly from the power supply to the ground when the two switch transistors are turned on simultaneously, the drive logic of switch transistors M1 and M2 are set to ensure the clocks do not overlap. However, such a clock setting may lead the switch transistors in series to be turned off simultaneously. In order to prevent such happening, the delay circuits Y1, Y2 having the same delay will be added to the circuitry.
Two possible states of the current flowing when switch transistors M1, M2 are concurrently turned off in the circuit in FIG. 1 are shown in FIG. 2, wherein different current states correspond to different input signals.
As shown in FIG. 2, when current Iout exists in the inductor, it will not disappear when both switch transistors M1 and M2 are turned off simultaneously (It is referred to be a “dead zone”), rather, it flows completely through the parasitic circuit (parasitic diode D1 or D2) of switch transistor M1 or M2, thereby the “overshoot” phenomena as shown in FIG. 3 will occur when the output current Iout>0 or Iout<0. Said “overshoot” phenomena means that the output voltage Vout may be higher than the power supply voltage Vcc or lower than zero during the dead zone time due to current Iout generated by the external inductor. When Iout>0 during the dead zone time, the output voltage Vout is lower than zero, the capacitor Cboot is charged continuously and thereby the voltage Vc of the capacitor Cboot becomes larger due to the “overshoot”. The turn-on resistance of the switch transistor M1 may be different according to different polarities of the output currents due to the fluctuation on the capacitor voltage Vc and thereby it leads to distortion. Wherein, the portions of the output voltage Vout, which are larger than the power supply voltage Vcc or lower than zero are called the “overshoot” voltage.
As shown in FIG. 3, Vout represents an output voltage of the amplifier; h represents the forward voltage drop of the body diodes D1, D2, that is, the depth of the “overshoot” voltage; and w is a non-overlapping time, that is, the width of the duration time of the “overshoot” voltage, also the duration time of the “dead zone”.
It can be seen from FIG. 3, when Iout>0, the overshoot voltage appears to be below the waveform of the output voltage Vout; while Iout>0, the “overshoot” voltage appears to be above the waveform of the output voltage Vout. The existence of the “overshoot” voltage makes the output voltage Vout be smaller than zero when Iout>0, and thereby it makes the voltage Vc on the capacitor Cboot be “over charged” and thus become larger, and further, the unexpected fluctuation of the voltage Vout will appear to further cause the change of the turn-on resistance of the switch transistor M1 and thereby it leads to distortion.